The body includes various passageways such as arteries, other blood vessels, and other body lumens. These passageways sometimes become occluded or weakened. For example, the passageways can be occluded by a tumor, restricted by plaque, or weakened by an aneurysm. When this occurs, the passageway can be reopened or reinforced, or even replaced, with a medical endoprosthesis. An endoprosthesis is typically a tubular member that is placed in a lumen in the body. Examples of endoprosthesis include stents and covered stents, sometimes called “stent-grafts”.
Endoprostheses can be delivered inside the body by a catheter that supports the endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen.
The expansion mechanism may include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries a balloon expandable endoprosthesis. The balloon can be inflated to deform and to fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter withdrawn.
In another technique, a self-expandable endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded, e.g., elastically or through a material phase transition. During introduction into the body, the endoprosthesis is restrained in a compacted condition on a catheter. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.
To support a passageway open, endoprostheses are made of materials, such as low-carbon, austenitic stainless steel or Nitinol (a nickel-titanium alloy), having appropriate mechanical properties, such as tensile strength and yield strength. An example of a suitable stainless steel is UNS S31673, which is similar to AISI 316L but having a higher chromium and nickel content range. UNS S 31673 has a general composition shown in Table 1:
TABLE 1Composition of UNS S31673ElementWeight PercentCarbon0.030 maximum Manganese2.00 maximumPhosphorus0.025 maximum Sulfur0.010 maximum Silicon0.75 maximumChromium17.00 to 19.00Nickel13.00 to 15.00Molybdenum2.25 to 3.00Nitrogen0.10 maximumCopper0.50 maximumIronBalance
where the chemical composition is maintained such that % Cr+(3.3)(X % Mo)≧26.0. Materials such as UNS S31673, however, can be relatively radiolucent. That is, the materials may not be easily visible under X-ray fluoroscopy, a technique used to locate and to monitor the endoprostheses during and after delivery. To enhance their visibility (e.g., by increasing their radiopacity), the endoprostheses can include a relatively radiopaque material, such as gold or platinum.